1. Field of the Invention
The invention relates to an evaporated fuel adsorbing mechanism for an internal combustion engine, which adsorbs evaporated fuel that flows back through an intake passage when the engine is stopped.
2. Description of the Related Art
When an internal combustion engine is stopped, air containing hydrocarbon (hereinafter referred to as HC) may flow back through an intake passage due to evaporation of fuel remaining near an intake port and an injector of the engine. Further, depending on the state of a cylinder when the engine is stopped, unburned air-fuel mixture remaining in a combustion chamber may flow back through the intake passage. Discharge of air containing HC into the atmosphere contributes to air pollution. To address this, an evaporated fuel adsorbing mechanism that includes an adsorbing member is available. The adsorbing member is provided downstream of a filter of an air cleaner to adsorb evaporated fuel using activated carbon or the like. With this arrangement, air containing HC passes through the adsorbing member when the air flows back to the upstream side of the intake passage. Thus, evaporated fuel is adsorbed by the adsorbing member.
In such an evaporated fuel adsorbing mechanism, the thickness of the adsorbing member may be increased to improve the capability of adsorbing the evaporated fuel. However, because air is introduced through the adsorbing member when the engine is operated, if the thickness of the entire adsorbing member is uniformly increased, a pressure loss may be greatly increased when the engine is operated.
Japanese Patent Application Publication No. 2003-193917 (JP-A-2003-193917) describes an evaporated fuel adsorbing mechanism that includes an adsorbing member. The adsorbing member is provided across the entire cross-section of a flow passage in an air cleaner. The thickness of the adsorbing member varies according to the concentration distribution of evaporated fuel. With this structure, the capability of adsorbing the evaporated fuel is improved, while the increase in the pressure loss during engine operation is suppressed as compared to the case where the thickness of the entire adsorbing member is uniformly increased.
In the evaporated fuel adsorbing mechanism described in JP-A-2003-193917, the thickness of the adsorbing member varies according to the concentration distribution of evaporated fuel. However, the resistance against the flow of intake air when the intake air passes through the adsorbing member during engine operation is equal to the resistance against the backflow of air when the air passes through the adsorbing member during engine stop. Therefore, the improvement in the capability of adsorbing the evaporated fuel results in the increase in the pressure loss during engine operation. Thus, the evaporated fuel adsorbing mechanism needs to be improved to improve the capability of adsorbing the evaporated fuel while suppressing the pressure loss during engine operation.